The real problem with running osx on an actual pc is that you may lose stability because it differs from the mac hardware base. But, since macs use pc hardware now. You can easily build your own computer buying and installing the same components that's inside a mac and install osx and have a similar experience to actually owning a real mac. That is the one thing you can do if you desire to run osx on what's essentially not a mac, or just build your own computer with your own components and install osx anyway (seems to work for many this way too, but you're best bet is to buy similar components that meet the mac hardware base when building your own pc to install osx onto).
A direct install of OSX doesn't work on a PC platform. It has been locked down to particular hardware configurations. However, that bit of code was cracked shortly after Apple announced they were moving to the x86 architecture, so its possible, but not straight out of the box so to speak.
Apple would make a killing if they dropped out of the hardware market and went into peripheral devices and software. They could distribute MacOS to its full potential, fix security vulnerabilities and compatibility issues that would undoubtably crop up, and focus on devices like iPods, phones, and PDA devices that rival anything currently on the market. Frankly, hardware is a waste of their time and it baffles me that they continue to dabble in the hardware line. I suspect they may be aware of the problems of a widespread distribution of the Mac OS without its A-list hardware and perhaps that is holding them back.
MacOS may also been a Unix-like OS but the MacOS X variants are actually built upon Unix layers beneath it. In essence, the OS is acting as an expanded graphical front-end. It's inherently more secure than Windows, or at least non-NT and pre-XP versions of Windows, but its not infallible. The primary reason for the lack of security exploits in MacOS is not that they aren't possible (weasling into the backbone of the OS is more than possible), but because it simply isn't worth the time to distribute malware to such a small percentage of the overall market.
Linux, in its present form, is not viable for mainstream users - primarily because it requires background reading. MacOS suffers because of the lack of mainstream programs available for it (not everyone has discovered open-source yet, and the majority of computer users won't touch the stuff). That leaves Windows - good old broken, bug-ridden, security sieve Windows. Unfortunately, it's relatively easy to use, requires no configuration by the end user, and will run every piece of software your average computer illiterate user can buy up off the shelf for the electronic wonders it promises.
Until the majority of computer users are actually educated about computing, Windows is the only viable operating system, as lamentable as that might be,